1. Field of the Invention
The present invention relates to a zoom lens suitable for imaging apparatuses equipped with a solid state image sensor such as a charged coupled device (CCD) and a complementary metal oxide semiconductor (CMOS).
2. Description of the Related Art
The pixel counts of solid state image sensors equipped on imaging apparatuses such as digital cameras and video cameras continue to increase. Higher performance enabling even finer features of a subject to be seen is also demanded of optical systems used on imaging apparatuses equipped with such solid state imaging sensors and numerous optical systems developed to meet such demands have appeared (see, for example, Japanese Patent No. 2899019, and Japanese Patent Application Laid-Open Publication Nos. H6-138390, 2010-237455, and 2005-227507).
High resolution imaging is also demanded of surveillance cameras to record images as evidence having high credibility for the purposes of crime prevention. Optical systems having a long focal length and achieving a bright image of the subject are also demanded to perform high resolution imaging.
Typically, surveillance cameras capture images using visible light during the day and near infrared light at night. Therefore, the optical system used in a surveillance camera has to accommodate a wide spectrum of wavelengths, ranging from visible light to near infrared. In general, in an optical system designed to use visible light, chromatic aberration especially occurs in the near infrared range and when imaging is performed at night in the near infrared range, the obtained images are out of focus. Thus, in particular, favorable correction of chromatic aberration occurring in the visible light range to the near infrared range is strongly demanded of optical systems used in surveillance cameras.
Furthermore, surveillance cameras used for crime prevention are preferably as small as possible so as to be inconspicuous. In addition, a compact size enabling housing within a small structure is demanded of the optical system used in surveillance cameras.
Although the optical system recited in Japanese Patent No. 2899019 has a zoom ratio on the order of 3 and offers favorable correction of aberration variations during zooming, the optical system is a dark lens having an F number on the order of 2.0 to 2.8. In addition, aberration of near infrared light is not corrected. Therefore, the optical system is not suitable as an optical system to be used for a surveillance camera.
Although the optical system recited in Japanese Patent Application Laid-Open Publication No. H6-138390 is compact having an overall length that is short, the optical system has a large F number, despite being a wide angle lens, and does not sufficiently correct chromatic aberration. Therefore, the optical system has a problem in terms of imaging performance with respect to near infrared light. In addition, the distance from the aperture stop to the image plane is long, making the focal length long and in turn, making accommodation in a small surveillance camera difficult.
Although the optical system recited in Japanese Patent Application Laid-Open Publication No. 2010-237455 is a zoom lens having a high zoom ratio and a relatively short overall length, making the optical system favorable for a small surveillance camera, the F number is large and chromatic aberration correction is insufficient. Thus, the optical system has a problem in terms of imaging performance with respect to near infrared light.
The optical system recited in Japanese Patent Application Laid-Open Publication No. 2005-227507 is a zoom lens having a high zoom ratio of 10 times, an F number on the order of 1.4 to 2.0, and favorable aberration correction of near infrared light. Nonetheless, the optical system has low imaging performance with respect to visible light and a long overall length, and therefore, is not suitable as an optical system for a small surveillance camera.